This patent pertains to the reduction of elastomeric particles to a very small size by abrading or grinding the particles between rotating milling wheels.
It has been found particularly advantageous in recycling and reusing elastomeric products to reduce the size of the rubber to very small size particles which are considered to be chemically more reactive and mechanically easier to dissolve into various mixes or recycled uses.
A wide range of rubber or synthetic rubber products may be so treated. Typical such materials include natural or synthetic rubber scrap, automotive tire scrap, and various polymers and plastics.
Various methods have been suggested in the prior art for reducing fine sized elastomeric particles as part of a reclaiming or recycling process including cryogenic cracking of the particles and various grinding or chopping methods.
However, the most productive method for producing particles in the very small range, defined as being particles that will pass a minus 50 mesh or finer, has been by grinding the rubber between horizontal grind stones in the manner well understood in the art and developed in the flour, paper pulp grinding industry and, most closely analogous to grinding rubber in the paint pigment compounding industry.
Previous methods for grinding rubber such are shown in for example U.S. Pat. No. 4,535,941 to Brubaker, et al which shows a method of horizontal grinding rubber pellets in which the pellets are introduced as a mixed rubber fluid slurry between and the milling wheels are pressed upon the pellets and slurry with considerable force in an effort to reduce them in a single pass to a fine state.
U.S. Pat. No. 4,625,922 to Brubaker shows another method for grinding rubber pellets utilizing elevated temperatures and pressures in an attempt to produce a fine rubber grind.
It has been found in practice that the above processes do not produce a uniformly fine grind and in fact, seldom produce particles in the claimed 50 micron range.
Further, adjustment of the grinding mills, which are the same mills widely used in the paint pigment compounding industry, is extremely critical under the methods previously disclosed. For instance, the first Brubaker patent requires pressure of over 2000 pounds be imposed between the milling wheels; the slightest cessation of seizure of slurry under these conditions would result in the instant contact of the milling wheels and the destruction of the same.
The attempt to simultaneously control motor current and stone pressure as variables results in wide swings and through put and an extreme variability in the quality of the produced product. It has proven in practice quite difficult to obtain sustained high uniform quality production rates using the processes heretofore described.